Ignition systems for igniting fuel in a turbine engine have been in wide use since the 1950's, and although a great variety of systems exist today, they have remained fundamentally unchanged since that time. One reason the design of ignition systems has not experienced fundamental changes over the years is that the design of a practical ignition system for turbine engines presents a significant challenge since the electronics of the system must operate reliably in severe environments --i.e., a wide range of temperatures, mixture ratios, humidities and pressures. An operating turbine may, for example, experience pressures as low as a few tenths of an atmosphere or as high as 10 atmospheres, and the ignitor must work at both extremes. For example, a flame-out during operation may necessitate re-ignition of the turbine fuel at a high altitude. At such high altitudes, the pressure is often only a few tenths of an atmosphere. Similarly, temperatures may range from extreme cold (e.g., -65.degree. F.), to very hot, for example when the high temperatures of the combustor soak the electronic module of the exciter in an ambient approaching 300.degree. F.
Typical ignition systems consist of three components: the exciter box, the ignition leads and the ignitor plug. The plug may be one of two types: air gap or semiconductor gap. The air gap plug is associated with high tension ignition systems because conditions of high pressure or wetness require very high voltage (e.g., 15 kV) to ionize the gap. The semiconductor plug is associated with low tension systems because it performs reliably with only 2-5 kV. However, a semiconductor-type plug generates a spark when supplied with only 1-2 kV (low tension), provided the voltage is applied for a relatively long period of time. In a semiconductor plug, the "semiconductor" is a material that provides an electrical shunt path across the air gap. This material conducts at a constant and low voltage (typically 1 kV), independently of pressure. The small current accompanying the low voltage helps to ionize the fuel mixture above the semiconductor surface, and the arc forms thereafter. Once the arc develops, the semi-conductor material does not conduct because the arc has much lower resistance, and the arc voltage is only about 30 volts. It is possible to use a semiconductor plug with high tension ignitions, but it is known in the art that this can cause excessive wear or even destruction of the semiconductor material. Even some low tension systems, which apply peak voltages of 5-8 kV, can damage the semiconductor element of the plug.
Categorizing ignition systems by the type of spark generated at the ignitor plug, there are two types of systems --bipolar and unipolar. In bipolar systems, the output is provided by an output transformer which steps up the relatively low voltage at an energy storage device to approximately 5-8 kV at the ignitor plug. Because an output transformer is utilized, the energy transferred to the the ignition plug is necessarily characterized by an alternating current which is typically of a relatively high frequency. The energy is delivered to the plug as a series of narrow pulses with high peak powers. As a result of delivery of the energy as a narrow pulse, a plug having a semiconductor gap is subjected to severe stress because the high voltages of the narrow pulses cause large, destructive currents in the semiconductor material prior to formation of an arc between the plug electrodes. Moreover, the components of the exciter and the ignition leads leading to the plug appear as loss elements in a bipolar discharge, thereby reducing the energy transferred to the spark gap for igniting the turbine fuel mixture. Also, the bidirectional nature of the arc current causes wear on both the inner and outer cylindrical electrodes of a semiconductor ignitor plug.
Because of their fundamentally different methods of generating a spark, unipolar ignition systems require substantially different design considerations than those applicable to bipolar systems. For example, a unipolar ignition does not use a transformer at its output and, therefore, it is not characterized by the same disadvantages created by the AC current in a bipolar ignition system. A unipolar ignition system produces a single pulse without oscillation which is trolled to have a 2-3 kV peak voltage. This "low tension" voltage is safe for the semiconductor plug, and the duration of the pulse is relatively long compared to the pulse of a bipolar ignition system. Furthermore, the multiple pulses in a bipolar system must each have a higher peak than the single peak in a unipolar pulse if the energy delivered is to be the same. Because of these higher peaks, the losses in the electronics and the ignition leads of the bipolar system are substantially greater than in an equivalent unipolar system. Also, a unipolar ignition is more amenable to the use of a solid state switch since the switch can be of a simpler nature because it is only required to handle direct current. Furthermore, a unidirectional arc current at the semiconductor plug can be directed to cause wear primarily on the larger (outer concentric) electrode, and alleviate erosion of the smaller (inner) electrode which always has less physical mass.
Although applicant is unaware of any quantitative comparative data, a substantial segment of the ignition system industry believes that a unipolar ignition system delivers to the gap of an ignitor plug a significantly greater percentage of the energy stored in an energy storage device. Assuming unipolar systems deliver a greater percentage of their stored energy to the arc, a unipolar system is more efficient and therefore more effective than the same sized bipolar system. Even though unipolar ignition systems offer various advantages over bipolar systems and have remained fundamentally unchanged over the years, it is still possible to improve the spark quality of such systems and thereby provide improved performance reliability.